Additive for nonaqueous electrolytic solution of electric double layer capacitor and nonaqueous electrolyte electric double layer capacitor

ABSTRACT

The invention is concerned with an additive for a non-aqueous electrolyte of an electric double layer capacitor having a high dissolving power of a support salt and a low viscosity and comprising a phosphazene derivative represented by the following formula (I):  
                 
 
(wherein R 1  is independently a halogen element or a monovalent substituent; and X is an organic group containing at least one element selected from the group consisting of carbon, silicon, nitrogen, phosphorus, oxygen and sulfur) as well as a non-aqueous electrolyte electric double layer capacitor comprising an electrolyte containing this additive and having excellent high-rate characteristics.

TECHNICAL FIELD

This invention relates to an additive for a non-aqueous electrolyte ofan electric double layer capacitor and a non-aqueous electrolyteelectric double layer capacitor obtained by adding the additive to anon-aqueous electrolyte, and more particularly to an additive for anon-aqueous electrolyte of an electric double layer capacitor having ahigh dissolving power of a support salt and a low viscosity and anon-aqueous electrolyte electric double layer capacitor having excellenthigh-rate characteristics (quick discharge-charge characteristics).

BACKGROUND ART

The non-aqueous electrolyte electric double layer capacitor is acondenser utilizing an electric double layer formed between apolarizable electrode and an electrolyte, which is a product developedand practiced in the 1970s, rendered into an infant stage in 1980s andcome into a growth developing stage in 1990s. In such a non-aqueouselectrolyte electric double layer capacitor, a cycle of electricallyadsorbing an ion on a surface of an electrode from an electrolyte ischarge-discharge cycle, which is different from a battery in which acycle of oxidation-reduction reaction accompanied with a mass transferis charge-discharge cycle. Therefore, the non-aqueous electric doublelayer capacitor is excellent in the instant charge-dischargecharacteristics as compared with the battery, and also the instantcharge-discharge characteristics are not substantially deteriorated evenin the repetition of the charge-discharge. Also, a simple and cheapelectric circuit is sufficient in the non-aqueous electrolyte electricdouble layer capacitor because there is no overvoltage in thecharge-discharge. Further, it has many merits that the residual capacityis easily understandable, and there is the temperature durablecharacteristic over a wide temperature range of −30 to 90° C., and thereis no pollution and the like as compared with the battery, so that itrecently comes under the spotlight as an earth-conscious and newenergy-storing product.

The electric double layer capacitor is an energy-storing devicecomprising positive and negative polarizable electrodes and anelectrolyte, in which positive and negative charges are oppositelyarranged in a contact interface between the polarizable electrode andthe electrolyte at a very short separating distance to form an electricdouble layer. The electrolyte plays a role as an ion source for theformation of the electric double layer, so that it is an importantsubstance dominating basic characteristics of the energy-storing devicelikewise the polarizable electrode. As the electrolyte have hithertobeen known aqueous electrolyte, non-aqueous electrolyte, solidelectrolyte and the like. From a point of improving the energy densityof the electric double layer capacitor, non-aqueous electrolytes capableof setting a high operating voltage particularly come under thespotlight and are putting into practical use. As such a non-aqueouselectrolyte is now practiced a non-aqueous electrolyte obtained bydissolving a solute (support salt) such as (C₂H₅)₄P.BF₄, (C₂H₅)₄N.BF₄ orthe like in an organic solvent having a high dielectric constant such asa carbonate (ethylene carbonate, propylene carbonate), γ-butyrolactoneor the like.

However, since the flash point of the solvent is low in thesenon-aqueous electrolytes, there is a problem that the risk is highbecause if the non-aqueous electrolyte electric double layer capacitoris fired by heat generation or the like, the electrolyte is ignited andthe flame is burnt out over the surface of the electrolyte. Also, thereis a problem that the non-aqueous electrolyte based on the organicsolvent is vaporized and decomposed accompanied with the heat generationof the non-aqueous electrolyte electric double layer capacitor togenerate a gas, and the non-aqueous electrolyte electric double layercapacitor is broken or fired by the generated gas to ignite thenon-aqueous electrolyte to burn out over the surface of the electrolyte.

On the contrary, there are known non-aqueous electrolyte electric doublelayer capacitors in which the risk of firing and igniting theelectrolyte is largely reduced by adding a particular phosphazenederivative to the electrolyte (see JP-A-2001-217152 andJP-A-2001-217154). In this electric double layer capacitor, theself-extinguishing property or flame retardance is given to thenon-aqueous electrolyte by a nitrogen gas or a halogen gas derived fromthe phosphazene derivative, whereby the risk of the fire and ignition isreduced. Also, phosphorus constituting the phosphazene derivative has anaction of suppressing the chain decomposition of a high molecular weightmaterial constituting the electric double layer capacitor, so that therisk of the fire and ignition is effectively reduced.

However, cyclic phosphazene derivatives disclosed in JP-A-2001-217152and JP-A-2001-217154 are very poor in the dissolving power of a supportsalt, so that when a greater amount of the cyclic phosphazene derivativeis added to the non-aqueous electrolyte, an ionic conductivity of theelectrolyte is lowered to lower the electric conductivity and hence thequick discharge characteristic and quick charge characteristic of theelectric double layer capacitor are poor. Recently, a quick start (quickdischarge) characteristic or an energy recovery (quick charge)characteristic in the braking is demanded in the electric double layercapacitor as an auxiliary power source for actively examined electriccars, so that the electric double layer capacitor obtained by adding agreat amount of the cyclic phosphazene derivative to the electrolyte iseffective in the application of the flame retardance but has a problemin the further stability of the quick charge-discharge as an electricdouble layer capacitor for the auxiliary power source of the electriccar. Also, such a tendency becomes remarkable at a temperature lowerthan room temperature, so that there is particularly a problem in thequick charge characteristic and quick discharge characteristic under alow temperature environment.

On the other hand, chain phosphazene derivatives disclosed inJP-A-2001-217152 and JP-A-2001-217154 are sufficient in the dissolvingpower of the support salt, but are somewhat higher in the viscosity ascompared with the cyclic phosphazene derivative, so that when such achain phosphazene derivative is added to the electrolyte, there is atendency of lowering the electric conductivity of the electric doublelayer capacitor. The lowering of the electric conductivity results inthe lowering of the above quick discharge and quick chargecharacteristics, so that the electric double layer capacitor having thechain phosphazene derivative added to the electrolyte has a problem inthe quick discharge and quick charge characteristics.

DISCLOSURE OF THE INVENTION

It is, therefore, an object of the invention to solve the problems ofthe conventional techniques and to provide an additive for a non-aqueouselectrolyte of an electric double layer capacitor having a highdissolving power of a support salt and a low viscosity and a non-aqueouselectrolyte electric double layer capacitor containing this additive ina non-aqueous electrolyte and having excellent high-rate characteristics(quick discharge-charge characteristics).

The inventors have made various studies in order to achieve the aboveobject, and found that a specified chain phosphazene derivatives has alow viscosity and a high dissolving power of a support salt and thatwhen such a phosphazene derivative is added to an electrolyte of anon-aqueous electrolyte electric double layer capacitor, the quickdischarge characteristic and quick charge characteristic of thiselectric double layer capacitor are improved, and as a result, theinvention has been accomplished.

That is, the additive for a non-aqueous electrolyte of an electricdouble layer capacitor according to the invention is characterized bycomprising a phosphazene derivative represented by the following formula(I):

(wherein R¹ is independently a halogen element or a monovalentsubstituent; and X is an organic group containing at least one elementselected from the group consisting of carbon, silicon, nitrogen,phosphorus, oxygen and sulfur).

In a preferable embodiment of the additive for the non-aqueouselectrolyte of the electric double layer capacitor according to theinvention, at least one of R¹s in the formula (I) is a halogen. As thehalogen, fluorine is particularly preferable.

In another preferable embodiment of the additive for the non-aqueouselectrolyte of the electric double layer capacitor according to theinvention, R¹ in the formula (I) is any one of an alkoxy group, aphenoxy group, an alkyl group, an aryl group, an acyl group, an aminogroup, an alkylthio group and an arylthio group.

In the other preferable embodiment of the additive for the non-aqueouselectrolyte of the electric double layer capacitor according to theinvention, X in the formula (I) is represented by any one of thefollowing formulae (IA), (IB), (IC), (ID) and (IE):

(in the formulae (IA), (IB), (IC), (ID) and (IE), R², R³, R⁴, R¹ and R⁶are independently a halogen element or a monovalent substituent; and Yis an organic group containing at least one element selected from thegroup consisting of oxygen, sulfur, carbon, silicon, nitrogen andphosphorus).

Also, the non-aqueous electrolyte electric double layer capacitoraccording to the invention is characterized by comprising a non-aqueouselectrolyte containing the above additive for the non-aqueouselectrolyte of the electric double layer capacitor and a support salt, apositive electrode, and a negative electrode.

In a preferable embodiment of the non-aqueous electrolyte electricdouble layer capacitor according to the invention, a content of thephosphazene derivative in the non-aqueous electrolyte is not less than 1volume %. At the moment, the content of the phosphazene derivative inthe non-aqueous electrolyte is preferably not less than 2 volume % froma viewpoint of the prevention of deterioration of support salt, furtherpreferably not less than 5 volume % from a viewpoint of the applicationof flame retardance to the electrolyte, particularly preferably not lessthan 10 volume % from a viewpoint of the application of incombustibilityto the electrolyte.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention will be described in detail below.

<Additive for Non-Aqueous Electrolyte of Electric Double LayerCapacitor>

The additive for the non-aqueous electrolyte of the electric doublelayer capacitor according to the invention comprises a phosphazenederivative represented by the formula (I). This phosphazene derivativeis high in the dielectric constant and high in the dissolving power ofthe support salt because it is a chain structure. Also, a compound inwhich a halogen having a high electronegativity is directly bonded tophosphorus or sulfur is very low in the viscosity. For this end, thenon-aqueous electrolyte containing such a phosphazene derivative is highin the ionic conductivity, and also the non-aqueous electrolyte electricdouble layer capacitor using such a non-aqueous electrolyte is excellentin the quick charge characteristic and quick discharge characteristic.

Further, when the above phosphazene derivative is included into theconventional non-aqueous electrolyte, it is possible to give anexcellent safety to the non-aqueous electrolyte electric double layercapacitor in the thermal runaway and reduce the risk of ignition or thelike under an action of nitrogen gas and phosphoric acid ester derivedfrom the phosphazene derivative. Also, since phosphorus has an action ofsuppressing the chain decomposition of the high molecular weightmaterial constituting the electric double layer capacitor, the safety ofthe electric double layer capacitor can be improved effectively.

Moreover, it is considered in the non-aqueous electrolyte electricdouble layer capacitor that a compound produced by decomposition orreaction of the electrolyte component or the support salt in thenon-aqueous electrolyte corrodes the electrode and the surroundingmembers or the amount of the support salt itself is decreased by such adecomposition or reaction to thereby bring about the troubles in theelectric characteristics and deteriorate the performances of thecapacitor. On the contrary, the phosphazene derivative suppresses thedecomposition or reaction of the electrolyte or support salt(particularly effectively acts to PF₆ salt) and contributes to thestabilization thereof. Therefore, it is possible to suppress thedeterioration while maintaining the electric characteristics by addingthe phosphazene derivative to the conventional non-aqueous electrolyte.

The viscosity at 25° C. of the phosphazene derivative represented by theformula (I) is not particularly limited as long as it is not more than4.5 mPa·s (cP), but it is preferably not more than 3.8 mPa·s (cP), morepreferably not more than 2.9 mPa·s (cP) from a viewpoint of theimprovement of the electric conduction and the improvement of lowtemperature characteristics. In the invention, the viscosity isdetermined by using a viscosity measuring device (R-type viscometerModel RE500-SL, made by Toki Sangyo Co., Ltd.) and conducting themeasurement at each revolution rate of 1 rpm, 2 rpm, 3 rpm, 5 rpm, 7rpm, 10 rpm, 20 rpm and 50 rpm for 120 seconds to measure a viscosityunder the revolution rate when an indication value is 50-60% as ananalytical condition.

The saturated dissolving amount of the support salt in the phosphazenederivative of the formula (I) is 1.5-3.0 mol per 1000 mL of thephosphazene derivative, for example, when the support salt is(C₂H₅)₄N.BF₄, and is preferable to be not less than 2.0 mol from aviewpoint of more preferably improving the electric conduction and lowtemperature characteristics.

In the formula (I), R¹ is independently a halogen element or amonovalent substituent. As the halogen element, fluorine, chlorine,bromine and the like are preferable, and among them, fluorine isparticularly preferable in a point that the viscosity is low. As themonovalent substituent are mentioned an alkoxy group, a phenoxy group,an alkyl group, an aryl group, an acyl group, an amino group, analkylthio group, an arylthio group and the like. Among them, the alkoxygroup, phenoxy group and amino group are preferable in a point that thepreparation is easy.

As the alkoxy group are mentioned methoxy group, ethoxy group, propoxygroup, butoxy group, an allyloxy group having a double bond, and analkoxy-substituted alkoxy group such as methoxyethoxy group,methoxyethoxyethoxy group or the like. As the phenoxy group arementioned phenoxy group, methylphenoxy group, methoxyphenoxy group andthe like. As the alkyl group are mentioned methyl group, ethyl group,propyl group, butyl group, pentyl group and the like. As the acyl groupare mentioned formyl group, acetyl group, propionyl group, butyrylgroup, isobutyryl group, valelyl group and the like. As the aryl groupare mentioned phenyl group, tolyl group, naphthyl group and the like. Asthe amino group are mentioned amino group, methylamino group,dimethylamino group, ethylamino group, diethylamino group, aziridylgroup, pyrrolidyl group and the like. As the alkylthio group arementioned methylthio group, ethylthio group, phenylthio group and thelike. As the arylthio group are mentioned phenylthio group, tolylthiogroup, naphthylthio group and the like.

A hydrogen element in the monovalent substituent may be substituted witha halogen element. In the formula (I), all of R¹s may be the same kindof the substituent, or some of them may be different substituents.Particularly, a case that at least one of R¹s is a halogen is preferablein a point that the flame retardance is improved, and further a casethat the halogen is fluorine is particularly preferable in a point thatthe viscosity is low.

In the formula (I), X is preferable to be an organic group having astructure represented by any one of the formulae (IA)-(IE). In theformulae (IA)-(IE), R²-R⁶ are independently a halogen element or amonovalent substituent. As R²-R⁶ are preferably mentioned the samehalogen elements and monovalent substituents as described in R¹ of theformula (I). R², R⁵ and R⁶ may be the same or different in the sameorganic group, or may be bonded to each other to form a ring. As Y arementioned, for example, NR group (R is an alkyl group, an alkoxyl group,a phenyl group or the like, which is so forth on), and a groupcontaining an element such as oxygen, sulfur, carbon, phosphorus,silicon or the like, and among them, NR group, oxygen and sulfur arepreferable.

<Non-Aqueous Electrolyte Electric Double Layer Capacitor>

The non-aqueous electrolyte electric double layer capacitor according tothe invention comprises a non-aqueous electrolyte containing theaforementioned additive for the non-aqueous electrolyte of the electricdouble layer capacitor and a support salt, a positive electrode and anegative electrode. The support salt contained in the non-aqueouselectrolyte can be selected from the conventionally known ones, but aquaternary ammonium salt is preferable in a point that the electricconduction or the like in the electrolyte is good. The quaternaryammonium salt is a solute of the electrolyte playing a role as an ionsource for the formation of the electric double layer. The quaternaryammonium salt capable of forming a polyvalent ion is preferable in apoint that it is possible to effectively improve the electriccharacteristics of the electrolyte such as electric conduction and thelike.

As the quaternary ammonium salt are preferably mentioned (CH₃)₄N.BF₄,(CH₃)₃C₂H₅N.BF₄, (CH₃)₂(C₂H₅)₂N.BF₄, CH₃(C₂H₅)₃N.BF₄, (C₂H₅)₄N.BF₄,(C₃H₇)₄N.BF₄, CH₃(C₄H₉)N.BF₄, (C₄H₉)₄N.BF₄, (C₆H₁₃)₄N.BF₄,(C₂H₅)₄N.ClO₄, (C₂H₅)₄N.AsF₆, (C₂H₅)₄N.SbF₆, (C₂H₅)₄N.CF₃SO₃,(C₂H₅)₄N.C₄F₉SO₃, (C₂H₅)₄N(CF₃SO₂)₂N, (C₂H₅)₄N.BCH₃(C₂H₅)₃,(C₂H₅)₄N.B(C₂H₅)₄, (C₂H₅)₄N.B(C₄H₉)₄, (C₂H₅)₄N.B(C₆H₅)₄ and the like.Also, a hexafluorophosphate in which an anion part of the quaternaryammonium salt (e.g. .BF₄, .ClO₂, .AsF₆ or the like) is replaced with.PF6 is preferable. Among them, a quaternary ammonium salt in whichdifferent alkyl groups are bonded to N atom is preferable in a pointthat the solubility can be improved by making a polarity large. Further,compounds represented by the following formulae (a)-(j) are preferablymentioned as the quaternary ammonium salt. In the formulae (a)-(j), Meis a methyl group and Et is ethyl group.

Among these quaternary ammonium salts, the salt capable of generating(CH₃)₄N⁺, (C₂H₅)₄N⁺ or the like as a cation is particularly preferablefrom a viewpoint of ensuring a high electric conduction. Also, the saltcapable of producing an anion with a small formula weight is preferable.These quaternary ammonium salts may be used alone or in a combination oftwo or more.

The phosphazene derivative represented by the formula (I) is high in thedissolving power of the support salt and low in the viscosity aspreviously mentioned, so that the ionic conductivity of the electrolyteis improved. As a result, the non-aqueous electrolyte electric doublelayer capacitor according to the invention using this electrolyte ishigh in the electric conductivity, excellent in the quick dischargecharacteristic and quick charge characteristic and also excellent in thelow temperature characteristics.

The amount of the support salt compounded to the non-aqueous electrolyteis preferably 0.5-1.5 mol, more preferably 0.5-1.0 mol per 1 L of theelectrolyte (solvent component). When the amount is less than 0.5 mol,the sufficient electric characteristics of the non-aqueous electrolytesuch as electric conduction and the like can not be ensured, while whenit exceeds 1.5 mol, the viscosity of the non-aqueous electrolyte risesand the quick charge-discharge characteristics and the low temperaturecharacteristics may be damaged.

The electrolyte for the non-aqueous electrolyte electric double layercapacitor according to the invention may contain an aprotic organicsolvent in addition to the support salt and the phosphazene derivativeof the formula (I). The aprotic organic solvent is preferable to have alow viscosity and a high electric conductivity from a viewpoint of theelectric characteristics.

The aprotic organic solvent is not particularly limited, but includesether compounds, ester compounds, nitrile compounds and the like.Concretely, there are preferably mentioned 1,2-dimethoxyethane,tetrahydrofuran, dimethyl carbonate, diethyl carbonate, ethylmethylcarbonate, ethylene carbonate, propylene carbonate, diphenyl carbonate,γ-butyrolactone, γ-valerolactone, acetonitrile and the like. Among them,cyclic ester compounds such as ethylene carbonate, propylene carbonate,γ-butyrolactone and the like; chain ester compounds such as dimethylcarbonate, diethyl carbonate, ethylmethyl carbonate and the like; andchain ether compounds such as 1,2-dimethoxyethane and the like arepreferable. The cyclic ester compound is preferable in a point that thedielectric constant is high and the solubility of the support salt isexcellent, and the chain ester and ether compounds are preferable in apoint that the viscosity is low and the viscosity of the electrolyte ismade low. They may be used alone or in a combination of two or more. Theviscosity at 25° C. of the aprotic organic solvent is not particularlylimited, but it is preferably not more than 5 mPa·s (cP), morepreferably not more than 3.0 mPa·s (cP).

The viscosity at 25° C. of the electrolyte in the non-aqueouselectrolyte electric double layer capacitor according to the inventionis preferably 1.0-5.0 mPa·s (cP), further preferably 1.0-4.0 mPa·s (cP).Since the electrolyte contains the above phosphazene derivative, theviscosity is low, and hence the non-aqueous electrolyte electric doublelayer capacitor according to the invention using such an electrolyte ishigh in the electric conductivity and excellent in the quick dischargecharacteristic and quick charge characteristic.

The content of the phosphazene derivative in the electrolyte of thenon-aqueous electrolyte electric double layer capacitor according to theinvention is preferable to be not less than 1.0 volume % from aviewpoint that the high-rate characteristics (quick charge-dischargecharacteristics) of the electric double layer capacitor are preferablyimproved. When the content of the phosphazene derivative is within theabove numerical range, the high-rate characteristics of the electricdouble layer capacitor can be preferably improved. Also, the phosphazenederivative is high in the wettability to the separator or the electrodein addition to the low viscosity, which is reflected in the improvementof the cell characteristics.

The content of the phosphazene derivative in the electrolyte of thenon-aqueous electrolyte electric double layer capacitor according to theinvention is preferable to be not less than 2 volume % from a viewpointthat “resistance to deterioration” can be preferably given to theelectrolyte. When the content of the phosphazene derivative is withinthe above numerical range, the deterioration can be preferablysuppressed. In this case, the “deterioration” means the corrosion of theelectrode and surrounding members and the decrease of the concentrationof the support salt accompanied therewith due to the formation of acompound through the decomposition or reaction of the electrolyte andsupport salt, and the effect of preventing the deterioration isevaluated by the following method of evaluating the stability.

-Stability Evaluating Method-

(1) After the preparation of the non-aqueous electrolyte containing thesupport salt, the water content is firstly measured. Then, theconcentration of hydrogen fluoride in the non-aqueous electrolyte ismeasured by NMR, GC-MS. Further, the color tone of the non-aqueouselectrolyte is visually observed, and thereafter the electric conductionis measured.

(2) After the non-aqueous electrolyte is left to stand in a globe boxfor 2 months, the water content and concentration of hydrogen fluorideare again measured, and the color tone is observed, and the electricconduction is measured. The stability is evaluated by the change ofthese measured numerical values.

Also, the content of the phosphazene derivative in the electrolyte ofthe non-aqueous electrolyte electric double layer capacitor according tothe invention is preferable to be not less than 5 volume % from aviewpoint that “flame retardance” is given to the electrolyte. Further,the content is preferable to be not less than 10 volume % from aviewpoint that “incombustibility” is given to the electrolyte. When thecontent of the phosphazene derivative is not less than 5 volume %, theelectrolyte becomes flame retardant, while when it is not less than 10volume %, the electrolyte becomes incombustible. At this moment, theflame retardance and incombustibility are defined by a method accordingto UL94HB method. In this case, when a test piece of 127 mm×12.7 mm isprepared by penetrating 1.0 mL of the electrolytes into anon-combustible quartz fiber and then the test piece is ignited under anatmosphere environment, a case that the ignited flame does not arrive ata line of 25 mm of the device and the ignition is not observed in thefalling object is the flame retardance, and a case that the ignition isnot caused (combustion length: 0 mm) is the incombustibility. In theinvention, the flame retardance and incombustibility are evaluated bymeasuring an oxygen index according to JIS K7201.

At this moment, the oxygen index means a value of lowest oxygenconcentration represented by a volume percentage required for continuingthe combustion of the material under given test conditions defined inJIS K7201. As the oxygen index becomes low, the risk of firing-ignitionis high, while as the oxygen index becomes high, the risk offiring-ignition is low, which means that “the safety is high”. Under theatmosphere condition, the oxygen index corresponds to 20.2 volume %, sothat the electrolyte having an oxygen index of 20.2 volume % means thatit burns in the atmosphere. As a result of the inventors' examination,it is confirmed that the electrolyte having an oxygen index of not lessthan 23 volume % has the flame retardance defined by the methodaccording to the UL94HB method and the electrolyte having an oxygenindex of not less than 25 volume % has the incombustibility defined bythe method according to the UL94HB method, so that the flame retardanceand incombustibility are evaluated by the measurement of the oxygenindex in the invention.

The positive electrode constituting the non-aqueous electrolyte electricdouble layer capacitor according to the invention is not particularlylimited, but is preferable to be usually a polarizable carbon-basedelectrode. As the polarizable electrode is preferable an electrodehaving such properties that the specific surface area and bulk gravityare usually large and the activity is electrochemically none and theresistance is small and the like. The polarizable electrode generallycomprises an activated carbon and may contain other components such asan electric conductive agent, a binder and the like, if necessary.

The material of the activated carbon used as the positive electrode isnot particularly limited, but includes preferably phenolic resin,various heat-resistant resins, pitch and the like. As the heat-resistantresin are preferably mentioned resins such as polyimide, polyamide,polyamideimide, polyether imide, polyether sulfon, polyether ketone,bismalimide triazine, aramide, fluorine resin, polyphenylene,polyphenylene sulfide and the like. They may be used alone or in acombination of two or more. As the shape of the activated carbon,powder, fibrous cloth and the like are preferable from a point that thespecific surface area is made higher to increase the charge capacity ofthe non-aqueous electrolyte electric double layer capacitor. Also, theactivated carbon may be subjected to a treatment such as heat treatment,drawing, high-temperature treatment under vacuum, rolling or the likefor the purpose of more increasing the charge capacity of the electricdouble layer capacitor.

The electric conductive agent used in the positive electrode is notparticularly limited, but includes graphite, acetylene black and thelike. The material of the binder is not particularly limited, butincludes resins such as polyvinylidene fluoride (PVDF),polytetra-fluoroethylene (PTFE) and the like.

As the negative electrode constituting the non-aqueous electrolyteelectric double layer capacitor according to the invention arepreferably mentioned the same polarizable electrodes as in the positiveelectrode.

The non-aqueous electrolyte electric double layer capacitor according tothe invention is preferable to comprise a separator, a currentcollector, a container and the like in addition to the positiveelectrode, negative electrode and the electrolyte, and further there canbe provided with various known members usually used in the electricdouble layer capacitor. At this moment, the separator is interposedbetween the positive and negative electrodes for the purpose ofpreventing the short-circuiting of the non-aqueous electrolyte electricdouble layer capacitor or the like. The separator is not particularlylimited, but there are preferably used known separators usually used asa separator for the non-aqueous electrolyte electric double layercapacitor. As the material of the separator are preferably mentionedmicroporous films, non-woven fabrics, papers and the like. Concretely,there are preferably mentioned non-woven fabrics, thin-layer films andthe like made of synthetic resin such as polytetrafluoroethylene,polypropylene, polyethylene or the like. Among them, a microporous filmof polypropylene or polyethylene having a thickness of about 20-50 μm isparticularly preferable.

The current collector is not particularly limited, but there arepreferably used known ones usually used as a current collector for thenon-aqueous electrolyte electric double layer capacitor. As the currentcollector, it is preferable to be excellent in the electrochemicallycorrosion resistance, chemically corrosion resistance, workability, andmechanical strengths and low in the cost, and a current collector layermade of aluminum, stainless steel, electrically conductive resin or thelike is preferable.

The container is not particularly limited, but there are preferablymentioned known ones usually used as a container for the non-aqueouselectrolyte electric double layer capacitor. As the material of thecontainer are preferable aluminum, stainless steel, electricallyconductive resins and the like.

The shape of the non-aqueous electrolyte electric double layer capacitoraccording to the invention is not particularly limited, but there arepreferably mentioned various known shapes such as cylinder type(cylindrical shape, square shape), flat type (coin type) and the like.The non-aqueous electrolyte electric double layer capacitor ispreferably used as an auxiliary power source for electric cars, as powersource for memory backup of various electronics, industrial instrumentsand airplane instruments and the like, for electromagnetic holding oftoys, cordless equipments, gas equipments, flash water heaters and thelike and for watches such as wrist watch, wall clock, solar watch, AGSwrist watch and the like.

In the non-aqueous electrolyte electric double layer capacitor accordingto the invention, the electric conductivity (specific conductance) ofthe electrolyte is not less than 5.0 mS/cm, preferably not less than 10mS/cm as an electric conductivity of a solution containing a supportsalt at a concentration of 1.0 mol/L. The non-aqueous electrolyteelectric double layer capacitor according to the invention is excellentin the high-rate characteristics (quick charge-dischargecharacteristics) because the electric conductivity is higher than thatof the conventional one as mentioned above. Moreover, the electricconductivity is a value obtained by measuring through an electricconductivity meter (trade name: CDM210, made by Radiometer Trading Co.,Ltd.) while applying a constant current of 5 mA to the electric doublelayer capacitor.

The following examples are given in illustration of the invention andare not intended as limitations thereof.

EXAMPLES

Using phosphazene derivatives shown in Table 1, the saturated dissolvingamount of tetraethyl ammonium fluoroborate [(C₂H₅)₄N.BF₄ (quaternaryammonium salt)] and viscosity are measured at 25° C. The results areshown in Table 1. In Table 1, phosphazene A is a compound shown by thefollowing formula (A), and phosphazene B is a compound shown by thefollowing formula (B), and phosphazene C is a compound shown by thefollowing formula (C), and phosphazene D is a compound shown by thefollowing formula (D), which are synthesized by the following methods.

(Synthesis of Phosphazene Derivative A)

A compound of the formulas (I), in which X is represented by the formula(IA) and all of R¹s and R²s are Cl and Y is oxygen, is reacted withsodium ethoxide in a toluene solvent at a temperature of −40° C. andsubjected to a molecular distillation to obtain a purified phosphazenederivative A.

(Synthesis of Phosphazene Derivative B)

Phosphorus trifluoride dichloride (PCl₂F₃) is reacted with diethylphosphorylamide in the absence of a solvent at room temperature andsubjected to a molecular distillation to obtain a purified phosphazenederivative B.

(Synthesis of Phosphazene Derivative C)

Phosphorus trifluoride dichloride (PCl₂F₃) is reacted with methanesulfonamide in the absence of a solvent at room temperature to obtain acompound of the formula (I) in which X is represented by the formula(IB) and all of R¹s are fluorine and R³ is methyl group. Then, thiscompound is reacted with pyrrolidine in a toluene solvent at roomtemperature and subjected to a molecular distillation to obtain apurified phosphazene derivative C.

(Synthesis of Phosphazene Derivative D)

Phosphorus trifluoride dichloride (PCl₂F₃) is reacted with acetoamide inthe absence of a solvent at room temperature to obtain a compound of theformula (I) in which X is represented by the formula (IC) and all of R¹sare fluorine and R⁴ is methyl group. Then, this compound is added withsodium phenoxide in an acetonitrile solvent at a temperature of −40° C.and subjected to a molecular distillation to obtain a purifiedphosphazene derivative D. TABLE 1 Saturated dissolving amount Viscosity(mol/L) (mPa · s) Phosphazene A 2.0 5.8 Phosphazene B 3.0 3.8Phosphazene C 3.0 3.3 Phosphazene D 3.0 2.9

As seen from Table 1, the phosphazene derivatives represented by theformula (I) are excellent in the dissolving power of the support saltand low in the viscosity as compared with the conventionally usedphosphazene derivative A.

Next, an electrolyte is prepared according to a compounding recipe shownin Table 2, and the viscosity of the electrolyte is measured at 25° C.and the oxygen index thereof is measured by the following method.

-Method of Measuring Oxygen Index-

The limit oxygen index is measured according to JIS K7201. A testspecimen is prepared by reinforcing a SiO₂ sheet (quartz filter paper,incombustible) of 127 mm×12.7 mm with a U-shaped aluminum foil so as torender into self-standing posture and impregnating 1.0 mL of theelectrolyte into the SiO₂ sheet. This test specimen is verticallyattached to a supporter for the test specimen so as to position at adistance of not less than 100 mm separated from an upper end portion ofa combustion cylinder (inner diameter of 75 mm, height of 450 mm,equally filled with glass particles of 4 mm in diameter over a regionranging from the bottom to 100±5 mm, a metal net placed thereon). Then,oxygen (equal to or more than JIS K1101) and nitrogen (equal to or morethan Grade 2 of JIS K1107) are flown into the combustion cylinder, whilethe test specimen is ignited in air (heat source is Class 1, No. 1 ofJIS K2240) to examine a combustion state. Moreover, the total flowingamount in the combustion cylinder is 11.4 L/min. This test is repeated 3times, and an average value thereof is determined.

Moreover, the oxygen index means a value of lowest oxygen concentrationrepresented by volume percentage required for continuing the combustionof the material under given test conditions defined according to JISK7201. In the invention, the limit oxygen index is calculated from alowest oxygen flowing amount required for continuously burning the testspecimen for not less than 3 minutes or continuing the combustion lengthof not less than 50 mm after the ignition and a nitrogen flowing amountat the time.Oxygen index=(oxygen flowing amount)/([oxygen flowing amount]+[nitrogenflowing amount])×100 (volume %)  Equation

A non-aqueous electrolyte electric double layer capacitor is prepared byusing the above electrolyte by the following method. With respect to theresulting electric double layer capacitor, the electric conduction,stability, low temperature characteristic and resistance todeterioration are measured and evaluated by the following evaluationmethods. These results are shown in Tables 2 and 3.

(Preparation of Electric Double Layer Capacitor)

An activated carbon (trade name: Kuractive-1500, made by Kurare ChemicalCo., Ltd.), acetylene black (electric conductive agent) andpolytetrafluoroethylene (PTFE)(binder) are mixed at a mass ratio of8/1/1/(activated carbon/acetylene black/PTFE) to obtain a mixture. 100 gof the resulting mixture is weighed and charged into a carbon pressurevessel of 20 mmφ and green-compacted at room temperature under apressure of 150 kgf/cm2 to prepare positive electrode and negativeelectrode (polarizable electrodes). A cell is assembled by using theresulting positive and negative electrodes, an aluminum metal plate(current collector)(thickness: 0.5 mm) and a polypropylene/polyethyleneplate (separator)(thickness: 25 μm) and sufficiently dried throughvacuum drying. This cell is impregnated with the electrolyte to preparea non-aqueous electrolyte electric double layer capacitor.

-Measurement of Electric Conduction-

The electric conduction is measured by using an electric conductivitymeter (trade name: CDM210, made by Radiometer Trading Co., Ltd.) whileapplying a constant current of 5 mA to the resulting electric doublelayer capacitor. The results are shown in Table 1.

-Evaluation of Stability-

With respect to the resulting non-aqueous electrolyte electric doublelayer capacitor, internal resistance at initial stage and aftercharge-discharge of 1000 cycles are measured to evaluate the long-periodstability. At this moment, the internal resistance (Ω) can be obtainedby a well-known method of measuring the internal resistance, forexample, a method in which a charge-discharge curve is determined tomeasure a deviation width of a potential accompanied with the stop ofcharge (charge rest) or the stop of discharge (discharge rest).

-Evaluation of Low Temperature Characteristics-

The internal resistance when the electric double layer capacitor isplaced at −20° C. is measured by an impedance analyzer.

-Evaluation of Resistance to Deterioration-

With respect to the resulting non-aqueous electrolyte, the resistance todeterioration is evaluated by measuring and calculating water content(ppm), concentration of hydrogen fluoride (ppm) and electric conductionjust after the preparation of the non-aqueous electrolyte and afterbeing left to stand in a globe box for 2 months in the same manner as inthe above method of evaluating the stability. Also, the change of colortone in the non-aqueous electrolyte is visually observed just after thepreparation of the non-aqueous electrolyte and after being left to standin the globe box for 2 months. TABLE 2 Low Stability of capacitortemperature Electrolyte Internal characteristic Aprotic Initialresistance Internal organic Oxygen Electric internal after resistancesolvent Phosphazene Support salt Viscosity index conductivity resistance1000 cycles at −20° C. (volume %) (volume %) (mol/L) (mPa · s) (volume%) (mS/cm) (Ω) (Ω) (Ω) Conventional GBL *1 phosphazene A (C₂H₅)₄N.BF₄4.5 22.6 9.5 0.18 0.19 0.25 Example 90 10 1.0 Example 1 GBL phosphazeneB (C₂H₅)₄N.BF₄ 4.1 24.8 11.2 0.13 0.13 0.19 90 10 1.0 Example 2 GBLphosphazene C (C₂H₅)₄N.BF₄ 4.0 25.2 11.7 0.12 0.12 0.18 90 10 1.0Example 3 GBL phosphazene D (C₂H₅)₄N.BF₄ 3.9 25.0 14.0 0.10 0.11 0.16 9010 1.0*1 γ-butyrolactone

TABLE 3 Evaluation of resistance to deterioration After being left tostand Initial for 2 months Electric HF Water Electric HF Water Changeconductivity concentration content conductivity concentration content ofcolor (mS/cm) (ppm) (ppm) (mS/cm) (ppm) (ppm) tone EvaluationConventional 0.18 0 2 0.18 0 2 none ◯: good Example Example 1 0.13 0 20.13 0 2 none ⊚: very good Example 2 0.12 0 2 0.12 0 2 none ⊚: very goodExample 3 0.10 0 1 0.10 0 1 none ⊚: very good

As seen from Table 2, the electrolyte in the non-aqueous electrolyteelectric double layer capacitor according to the invention is low in theviscosity as compared with the conventional electrolyte, and thenon-aqueous electrolyte electric double layer capacitor according to theinvention using this electrolyte is high in the electric conductivity ascompared with the conventional battery. As a result, the non-aqueouselectrolyte electric double layer capacitor according to the inventionis excellent in the quick charge characteristic and the quick dischargecharacteristic. Also, the characteristics as an electric double layercapacitor and the resistance to deterioration in the non-aqueouselectrolyte electric double layer capacitor according to the inventionare equal to or more than those of the conventional battery.Furthermore, the non-aqueous electrolyte electric double layer capacitoraccording to the invention is high in the oxygen index and high in thesafety of the electrolyte.

INDUSTRIAL APPLICABILITY

According to the invention, there can be provided an additive for anon-aqueous electrolyte of an electric double layer capacitor having ahigh dissolving power of a support salt and a low viscosity. Also, thenon-aqueous electrolyte electric double layer capacitor according to theinvention obtained by adding this additive to the electrolyte is high inthe electric conductivity and excellent in the quick dischargecharacteristic and the quick charge characteristic.

1. An additive for a non-aqueous electrolyte of an electric double layercapacitor characterized by comprising a phosphazene derivativerepresented by the following formula (I):

(wherein R¹ is independently a halogen element or a monovalentsubstituent; and X is an organic group containing at least one elementselected from the group consisting of carbon, silicon, nitrogen,phosphorus, oxygen and sulfur).
 2. An additive for a non-aqueouselectrolyte of an electric double layer capacitor according to claim 1,wherein at least one of R¹s in the formula (I) is a halogen.
 3. Anadditive for a non-aqueous electrolyte of an electric double layercapacitor according to claim 2, wherein the halogen is fluorine.
 4. Anadditive for a non-aqueous electrolyte of an electric double layercapacitor according to claim 1, wherein R¹ in the formula (I) is any oneof an alkoxy group, a phenoxy group, an alkyl group, an aryl group, anacyl group, an amino group, an alkylthio group and an arylthio group. 5.An additive for a non-aqueous electrolyte of an electric double layercapacitor according to claim 1, wherein X in the formula (I) isrepresented by any one of the following formulae (IA), (IB), (IC), (ID)and (IE):

(in the formulae (IA), (IB), (IC), (ID) and (IE), R², R³, R⁴, R⁵ and R⁶are independently a halogen element or a monovalent substituent; and Yis an organic group containing at least one element selected from thegroup consisting of oxygen, sulfur, carbon, silicon, nitrogen andphosphorus).
 6. A non-aqueous electrolyte electric double layercapacitor comprising a non-aqueous electrolyte containing an additivefor a non-aqueous electrolyte of an electric double layer capacitor asclaimed in any one of claims 1 to 5 and a support salt, a positiveelectrode, and a negative electrode.
 7. A non-aqueous electrolyteelectric double layer capacitor according to claim 6, wherein a contentof the phosphazene derivative in the non-aqueous electrolyte is not lessthan 1.0 volume %.
 8. A non-aqueous electrolyte electric double layercapacitor according to claim 7, wherein the content of the phosphazenederivative in the non-aqueous electrolyte is not less than 2 volume %.9. A non-aqueous electrolyte electric double layer capacitor accordingto claim 8, wherein the content of the phosphazene derivative in thenon-aqueous electrolyte is not less than 5 volume %.
 10. A non-aqueouselectrolyte electric double layer capacitor according to claim 9,wherein the content of the phosphazene derivative in the non-aqueouselectrolyte is not less than 10 volume %.